Rose Mosaic Virus Disease

Many of you know that the primary reason we grow roses at Florida Southern College is our involvement in indexing and heat-treating roses for rose mosaic disease. While we have had articles about the subject in numerous other publications over the past decade, I’ve not mentioned the subject in The Cherokee Rose, nor has there been any extensive discussion of the subject at any of our meetings. Yet it is a subject I believe to be quite important, particularly in that a grower, through ignorance of the problem, could introduce a viral infection to an antique rose which may have survived hundreds of years without the disease. A few simple precautions could have prevented the infection. Also, some old rose nurseries are notorious for shipping virus-infected plants, while others have made a great effort to provide virus-free bushes. I certainly commend (and recommend) the latter group.

The following is an updated version of a paper I presented to the Florida State Horticultural Society, in 1985. The Citrus Institute of Florida Southern College initiated a program to rid infected rose plants of rose mosaic (RM) disease in 1984. This paper will describe the disease, its effects on rose plants and their culture, and the heat therapy program at Florida Southern College.

Rose mosaic is a disease caused by a virus complex infecting cultivated roses (Rosa spp. and hybrids). Cochran 3 reported that by 1970, most of the garden roses in the United States were infected. Since then, heat therapy programs have been initiated at the Oregon State University and the University of California at Davis, as well as by Bear Creek (parent company of Jackson & Perkins Roses and Armstrong Roses). The Oregon State program is now nearly defunct. Some commercial rose nurseries have made use of those programs and now offer virus-free plants for sale. However, many nurseries have not made any attempt to provide healthy plants, and a large percentage of the roses grown and sold in Florida are infected.

Florida nurseries using Fortuniana as a rootstock are at a particular disadvantage, since scion-source plants of new cultivars are received from a single source, usually on Dr. Huey rootstock, from California. If these original plants are infected, then all plants subsequently produced on Fortuniana rootstock will be infected. In recent years virtually all new cultivars, including the All America Rose Selections (AARS) winners, have been infected with RM when received by the Florida nurserymen (personal communication from several nurserymen, diagnosed by leaf symptoms.) The disease also may be spread to other cultivars through the use of infected rootstock. No source of indexed virus-free Fortuniana plants has been available until recently, although some propagators have been quite conscientious about selecting their rootstock cuttings only from plants which have never shown symptoms of RM.

Since RM is not fatal to the plant and often has no obvious detrimental effect on a rose, nurserymen and rosarians tend to be unconcerned about the problem. When leaf symptoms appear on a plant, the affected branch is pruned off, temporarily ridding the plant of its symptoms. If (as many growers believe) the only effect of RM were an occasional chlorotic or disfigured leaf, there would be little cause for concern about the disease.

The symptoms are highly variable among rose cultivars and are strongly influenced by weather and growing conditions. Infected plants may appear to be quite healthy for much of the year, and any symptoms which do appear may be attributed to other causes, such as spray burn, nutrient deficiencies, high temperature, or poor horticultural practices. It has been suggested that the “deterioration” which often occurs in rose cultivars several years after their introduction may be a result of virus infection 1.

Etiology Rose mosaic is a complex of several viruses which cause similar symptoms in rose plants. The most important of these in the United States is prunus necrotic ringspot virus, a common disease of stone fruit trees 5. Of lesser importance in the USA are apple mosaic virus and arabis mosaic virus. There may be additional viruses involved in the RM complex 6. Several other virus diseases of rose are quite distinct from RM and will not be considered in this paper. These include rose wilt, rose leaf curl, rose streak, rose rosette, and rose spring dwarf.

Means of Transmission RM is believed to be non-contagious in the field, except possibly through rare natural root grafts. There is no evidence that it ever spreads naturally in the garden or nursery, or through pollen, seed, or seedlings 2. Extensive tests also have failed to transfer RM mechanically (e.g., on pruning tools, grafting knives, etc.) 3. The only known means for transmitting the disease is by vegetative propagation. Cuttings rooted from infected plants, or budded plants produced from infected scions or rootstocks, will be infected in virtually every case. The disease is systemic, so the entire plant is infected, whether or not all of the branches show symptoms. A plant which is infected at the time of propagation will remain infected throughout its life, and a healthy plant at the time of propagation should remain healthy for its entire life, unless an infected scion is budded or grafted onto it. It is probable that the disease was transferred to roses originally from one of the stone fruits, by graftage 4. It then spread from one rose cultivar to another through infected rootstocks. Two nursery practices contributed to the rapid spread of the disease in the United States:

Collecting scion wood for next year’s crop from this year’s budded plants in the production field, rather than from a separate, disease-free, scion-source garden 4.

Collecting rootstock cuttings from suckers on budded plants in the production field, rather than from a non-budded, disease-free rootstock planting. In Europe, where rootstock plants are usually produced from seed, RM remains quite rare 3.

Leaf Symptoms Leaf symptoms of RM are highly variable, often making diagnosis difficult. Some rose cultivars show strong symptoms, while others may be nearly symptomless. Most cultivars will be symptomless for at least part of the year. The most severe symptoms usually are seen during cool weather, in the spring, and are much less severe during the summer months. Some leaves may show “vein-banding”, in which the veins are bright orange or yellow, on a green background. Other leaves may show a bright yellow or white “oak leaf” or “mosaic” pattern . A very faint “watermark” chlorosis is common on the leaves of some cultivars .

These symptoms often fade as the leaf ages and may disappear completely. The chlorotic patterns associated with RM usually do not closely resemble any mineral nutrient deficiency or herbicide toxicity pattern and are reasonably reliable for diagnosing RM. The absence of any obvious symptoms is normal, and is no guarantee of freedom from RM; some infected cultivars seldom show symptoms, but their performance may be impaired. The Heat Therapy Program at Florida Southern College Florida Southern College’s heat therapy program was initiated with the following goals:

To produce rootstock plants adapted to rose culture in Florida that are known to be free of RM, particularly Fortuniana and Fun Jwan Lo .

To provide propagating material of rootstock and scion cultivars to nurseries interested in cooperating with the program, thus enabling Florida residents to purchase disease-free plants on desirable rootstocks.

To maintain a RM-free garden for the preservation of healthy germplasm of the treated cultivars. The heat therapy procedures are similar to those employed by the programs at the Oregon State University and the University of California at Davis. Infected scionwood is budded or grafted to Fortuniana rootstock and grown to a 2-gallon size plant. The potted plant is placed in a controlled-environment chamber, where the temperature is held at a constant 38°C (100°F) for 21-35 days. The heat treatment does not cure the plant, but RM-free material can be obtained as follows: Axillary buds from the treated plant are budded onto RM-free rootstocks. Most of the axillary buds on the heat-treated plant will be free of RM. Once the new budlings are growing, they must be tested to insure freedom from RM, a process known as “indexing.”

We use three indexing methods:

Mme. Butterfly – Buds from the plant to be tested are budded to established plants of virus-free Mme Butterfly an older Hybrid Tea which shows brilliant mosaic symptoms when first infected. This is usually done in the autumn. The plant is allowed to grow a new flush of Mme. Butterfly leaves during the spring, and those leaves are observed for symptoms.

Shirofugen – Buds from the plant to be tested are budded to branches of Shirofugen a Japanese flowering cherry tree. Roses and cherries are not graft-compatible, so the graft always dies. If the bud was not infected, the cherry branch heals over, cleanly. But if the rose bud contained mosaic virus, the virus will be transferred to the cherry branch, which will react by producing a sticky, gummy oozing sap, and the area around the graft union will die. Cherry trees don’t grow well in Central Florida, so we contract with the University of California to do this test for us. We ship them budwood to be tested, in June.

ELISA — Enzyme-Linked Immunosorbent Assay is a laboratory method, using rabbit antibodies. It is a quick (less than one day) laboratory test, and not only tells whether any virus is present, but can often determine exactly which virus, and sometimes even which specific strain of a virus, is present. We contract with the Washington State University, to do this test, sending them leaf samples in cool weather.

The program at Florida Southern College is now nearly 10 years old. We have heat-treated and/or indexed hundreds of varieties, and now maintain more than 350 virus-free scion varieties, including around 200 old garden roses. We also have virus-free rootstocks, including Fortuniana Fun Jwan Lo and Dr. Huey. Mosaic-free plant material is available to commercial nurseries for propagation, and it is through our cooperating nurseries that mosaic-free plants are available to the public.

Summary and Conclusions Rose mosaic disease currently infects a large percentage of the roses grown in Florida, and throughout the United States. While hobbyist growers and most nurseries lack the facilities to rid plants of the disease, cultivars can be freed of RM by a simple heat treatment program. Florida Southern College is engaged in such a program, and offers virus-free material to commercial nurseries, to the extent that time and facilities will permit. Since RM is believed never to spread by natural means, there is no legitimate excuse for its continued existence in American rose nurseries and gardens. While RM is not deadly or otherwise devastating to a rose bush, improved growth and more flowers of higher quality may be expected from disease-free plants, so it is to a grower’s advantage to seek out plants known to be free of the disease.

Thomas, B.J. 1984. Epidemiology of three viruses infecting the rose in the United Kingdom. Ann. Appl. Biol. 105:213-222.

As an addendum to the above article, here are a few thoughts on what rose mosaic means for the home gardener:

You can’t cure it in your garden, but it is not going to spread from bush to bush. So there is no great need to dig up and destroy an infected bush. However, if you can find a virus-free plant of the same variety, you might want to consider replacing the bush, to gain more vigor and greater flower production.

If you do your own budding or grafting, remember that those procedures spread the disease, so try to use virus-free scion wood and virus-free rootstocks. If you root cuttings of Fortuniana that sprouted out from the base of a grafted bush, remember that those cuttings will contain the virus if the original bush was infected. Also, any scions collected from an infected bush will produce infected plants, when propagated.

Remember that a complete lack of symptoms (i.e., a healthy looking bush) is the normal situation for an infected plant. Just because a plant appears to be healthy, even for several years, is no guarantee that it is indeed virus-free. Only indexing can tell you for sure.

One of the major reasons so many nurseries are “cleaning up” their stock, in recent years, is customer demand. Please support and commend nurseries that produce clean plants. Encourage nurseries who don’t, to begin growing virus-free roses. If they know it is important to you, the customer, they will likely respond favorably.

While I am not aware of any nursery which sells only virus-free plants, most of the nursery-members of the CFHRS do grow at least some clean varieties, and will gladly tell you, if you ask, which of their stock is clean. It will be quite a long time until all of the commercially propagated heritage roses can be cleaned up, but we’ve made a good start. Here’s a partial listing of older roses available from our program, through retail nurseries: